Instructions

First, collect the data. We have made observations of delta
Cephei on four nights (10/05, 10/26, 11/02, & 11/09); weather
permitting, we will try to make two more. Copy the dates, times, and
average magnitudes for each night from the observing worksheet to the
table above. Eight more observations (made by Mike Nassir and Roberto
Mendez) will be provided in class; there will be one set of four
observations on yellow paper, and four individual observations on
white paper. Make sure the dates of the observations on white paper
are all different. Copy the dates, times, magnitudes, and observer's
initials to the table above.

Second, plot magnitude versus date on the top graph provided
with this handout. Plot each magnitude and date as accurately as you
can, taking account of the time the observations were made. Don't
expect to see any obvious pattern in this graph; the star has gone
through many cycles this semester, and our observations catch
the star at random points in its cycle.

Third, for each observation, calculate the number of days
since 9/01 at 0:00 HT. Express your answer to the
nearest tenth of a day, and write this number in the Days
column.

Example: The observation on 10/09 at 21:45 HT was made
30 + 9&nbsp+ (21÷24)&nbsp+ (45÷1440)
= 39.90625 days after 9/01 at 0:00 HT, so write 39.9 in the
Days column.

Fourth, for each observation, divide the number of days
since 9/01 at 0:00 HT by the period of delta Cephei,
which is 5.37 days, and keep only the part after the decimal point.
Write this number in the Phase column, rounding it to two
significant figures.

Example: For the observation on 10/09, we have
39.9 ÷ 5.37 = 7.43017, so write
0.43 in the Phase column.

Fifth, plot magnitude versus phase on the bottom graph
provided with this handout. Plot each point twice; once with the
phase you computed, and again after adding 1 to the phase.

Example: The observation on 10/09 yielded a magnitude
of 4.30 and a phase of 0.43, so plot points with phases of 0.43
and 1.43 at this magnitude.

As you plot these points, you should gradually see a regular
pattern of variation emerge; this is the ``light curve'' of delta
Cephei (repeated because each point is plotted twice). Why does this
graph display a regular pattern, while the first one did not? The
answer has to do with the fact that a star like delta Cephei varies in
a regular and predictable way, repeating the same behavior again and
again. Because of this, we can make observations over a long period,
calculate where in the star's cycle each observation falls, and put
them together to see how the star varies over the course of its
cycle.